The present invention relates to source coding systems utilising high frequency reconstruction (HFR) such as Spectral Band Replication, SBR [WO 98/57436] or related methods. It improves performance of both high quality methods (SBR), as well as low quality copy-up methods [U.S. Pat. No. 5,127,054]. It is applicable to both speech coding and natural audio coding systems. Furthermore, the invention can beneficially be used with natural audio codecs with- or without high-frequency reconstruction, to reduce the audible effect of frequency bands shut-down usually occurring under low bitrate conditions, by applying Adaptive Noise-floor Addition.
The presence of stochastic signal components is an important property of many musical instruments, as well as the human voice. Reproduction of these noise components, which usually are mixed with other signal components, is crucial if the signal is to be perceived as natural sounding. In high-frequency reconstruction it is, under certain conditions, imperative to add noise to the reconstructed high-band in order to achieve noise contents similar to the original. This necessity originates from the fact that most harmonic sounds, from for instance reed or bow instruments, have a higher relative noise level in the high frequency region compared to the low frequency region. Furthermore, harmonic sounds sometimes occur together with a high frequency noise resulting in a signal with no similarity between noise levels of the highband and the low band. In either case, a frequency transposition, i.e. high quality SBR, as well as any low quality copy-up-process will occasionally suffer from lack of noise in the replicated highband. Even further, a high frequency reconstruction process usually comprises some sort of envelope adjustment, where it is desirable to avoid unwanted noise substitution for harmonics. It is thus essential to be able to add and control noise levels in the high frequency regeneration process at the decoder.
Under low bitrate conditions natural audio codecs commonly display severe shut down of frequency bands. This is performed on a frame to frame basis resulting in spectral holes that can appear in an arbitrary fashion over the entire coded frequency range. This can cause audible artifacts. The effect of this can be alleviated by Adaptive Noise-floor Addition.
Some prior art audio coding systems include means to recreate noise components at the decoder. This permits the encoder to omit noise components in the coding process, thus making it more efficient. However, for such methods to be successful, the noise excluded in the encoding process by the encoder must not contain other signal components. This hard decision based noise coding scheme results in a relatively low duty cycle since most noise components are usually mixed, in time and/or frequency, with other signal components. Furthermore it does not by any means solve the problem of insufficient noise contents in reconstructed high frequency bands.
The present invention addresses the problem of insufficient noise contents in a regenerated highband, and spectral holes due to frequency bands shut-down under low-bitrate conditions, by adaptively adding a noise-floor. It also prevents unwanted noise substitution for harmonics. This is performed by means of a noise-floor level estimation in the encoder, and adaptive noise-floor addition and unwanted noise substitution limiting at the decoder.
The adaptive Noise-floor Addition and the Noise Substitution Limiting method comprise the following steps:
At an encoder, estimating the noise-floor level of an original signal, using dip- and peak-followers applied to a spectral representation of the original signal;
At an encoder mapping the noise-floor level to several frequency bands, or representing it using Linear Predictive Coding (LPC) or any other polynomial representation;
At an encoder or decoder, smoothing the noise-floor level in time and/or frequency;
At a decoder, shaping random noise in accordance to a spectral envelope representation of the original signal, and adjusting the noise in accordance to the noise-floor level estimated in the encoder;
At a decoder, smoothing the noise level in time and/or frequency;
Adding the noise-floor to the high-frequency reconstructed signal, either in the regenerated high-band, or in the shut-down bands.
At a decoder, adjusting the spectral envelope of the high-frequency reconstructed signal using limiting of the envelope adjustment amplification factors.
At a decoder, using interpolation of the received spectral envelope, for increased frequency resolution, and thus improved performance of the limiter.
At a decoder, applying smoothing to the envelope adjustment amplification factors.
At a decoder generating a high-frequency reconstructed signal which is the sum of several high-frequency reconstructed signals, originating from different lowband frequency ranges, and analyzing the lowband to provide control data to the summation.